Dear Kim Dae Seong , the reason why polysulfides (PS) need to be dissolved, is because they are non-conductive products that will block the electrochemical reduction of elemental sulfur in case of their "non-dissolution". Imagine that on top of the sulfur layer, there will be more layers of PS that will not allow the transfer of electrons, hence the necessity of their dissolution in order to allow contact between sulfur and the conductive carbon. Quoted from Sheng S. Zhang. Journal of Power Sources 231 (2013) 153-162 : "Since both elemental sulfur and its reduction products are non-conductive, the reduction of sulfur and PS only can take place on the surfaces of conductive carbon. By dissolution, the soluble PS dissolves into electrolyte solution, which leaves remaining sulfur exposed to the conductive carbon so that the reduction can progressively move forward".

However this doesn't mean that the PS need to be dissolved in the electrolyte solution because it will create a shuttle effect and lowers the performance of the Li-S battery. So the ideal solution is to "trap" these PS at the moment of their formation AND allowing at the same time lithium ions to react with the elemental sulfur (since the PS are not forming a layer on top of the sulfur). That's why PS are inevitable and essential; and many researchers are trying to handle this issue by using materials such as carbon doped heteroatoms, conductive polymers and different structure of sulfur compounds (yolk-shell; core-shell) etc... to physically and chemically trap PS while allowing the proceeding of the electrochemical reactions of the elemental sulfur. To learn more about this issue you can refer to the following articles:

Article Recent advances in shuttle effect inhibition for lithium sul...

Article Sulphur–TiO2 yolk–shell nanoarchitecture with internal void ...

I really appreciate for your honest answer!

now, I exactly understood it!

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